1. The present invention relates to a mobile communication system and, more particularly, to a mobile communication system in which a handover of a mobile station is performed when a completed call is originated while the mobile station is moving.
2. Description of the Related Art
In recent years, terminals of mobile communication system are rapidly spreading due to liberalization of the market and application of high-level digital transmission techniques. Such terminals are further spread in response to development and widespread use of a system such as a personal handy phone system (PHS) which offers a low-cost mobile communication service. The development is directed to an application for satisfying versatile needs. Accordingly, when such a terminal is used in a highspeed traffic system and when a completed call originates while a mobile station is moving, the number of times of handover of the call is increased as speed of movement of the mobile station increases.
In a conventional mobile communication system, when the completed call is originated while a mobile station is moving, a mobile station (hereinafter referred to as a transferor cell) in which the completed call is currently originating and a radio base station (hereinafter referred to as a transferor base station) form a cell (radio zone) in which the mobile station is located. The mobile station and the base station cooperate with each other so as to measure transmission quality of the radio transmission paths for adjacent cells or peripheral cells (hereinafter referred to as the adjacent cells as a whole) which are candidates for the mobile station to be moved. The measurement based on an electric field intensity in addition to measurements of transmission quality in the transferor cell. The transferor base station compares the measured transmission qualities, and performs a radio channel setting control operation necessary for resuming the current communication when the transmission quality of the transferor cell is higher than that of the adjacent cells. However, on the contrary, if the transmission quality of the adjacent cells is higher than that of the transferor cell, the transferor base station determines the candidate of the adjacent cell (hereinafter referred to as an adjacent cell) to which the mobile station will be moved by a handover based on a descending order of transmission quality. Additionally, the transferor base station sends a notification that the radio base station forming the adjacent cell (hereinafter referred to as candidate base station) is set as a destination for handover of the call. The notification includes necessary information as the transferee base station.
On the other hand, upon recognition of the notification, the candidate base station seeks a radio channel which is currently available, and assigns the available channel to a call for the mobile station entering the candidate base station. The candidate base station sends the result to the transferor base station, and starts a communication test through the assigned channel. It should be noted that the radio channel thus assigned is hereinafter referred to as a candidate channel.
Additionally, the transferor base station discriminates the notified candidate channel, and sends discrimination information of the radio channel (candidate channel) to the mobile station.
Thereafter, the mobile station performs the above-mentioned communication test by cooperating with the candidate base station through the radio channel (candidate channel) indicated by the discrimination information. If the communication test ends in a normal condition, the result is sent to the transferor base station.
Upon the receipt of the completion of the communication test from the mobile station, the transferor base station transfers the result to the transferee base station, and sends a notification to an exchange (hereinafter referred to as a transferor exchange) which controls the transferor base station. The notification indicates the discrimination information (such as a mobile station number) corresponding to the call and the discrimination information of the candidate channel.
Thereafter, upon recognition of the abovementioned notification, the transferee base station performs a process for connecting the candidate channel to a corresponding communication path of the call by cooperating with an exchange (hereinafter referred to as a transferee exchange) which is controlled by the transferee exchange. The transferee base station sends to the transferor base station a notification which indicates a completion of such process.
Upon recognition of the notification, the transferor base station releases the radio channel which has been assigned to the corresponding call.
As a result, the mobile station, which is moving to one of the candidate cells while maintaining communication after the completed call is originated, can acquire such a candidate cell and a call assigned to the candidate cell. Additionally, a radio transmission path used for a communication is positively reserved as long as a channel setting control is performed by cooperation of the transferor base station and the candidate base station based on the above-mentioned procedure.
Additionally, in the conventional PHS, a mobile station automatically recognizes that the mobile station is positioned close to the outside of the transferor cell, and selects the candidate cell. Then, the mobile station directly sends to the candidate base station, which forms the candidate cell a request for handover (hereinafter referred to as a handover request).
The candidate base station achieves the handover by performing a process equivalent to the process performed by the above-mentioned conventional system in cooperation with the transferor base station.
Accordingly, a handover can be achieved in a city area in which a transmission characteristic of the transmission path rapidly deteriorates due to buildings located between the mobile station and the transferor base station even when the mobile station, in which the completed call originates, is located within a short distance from the transferor base station.
However, in the above-mentioned conventional system, the selection of the candidate cell is not performed until the mobile station reaches a position close to the outside of the transferor cell even when the transferor cell and the adjacent cell overlap with each other. Thus, when the mobile station moves at an extremely high speed or the transmission characteristic of the transmission path intensely fluctuates, there is a high probability that the handover is not normally performed.
Additionally, in the PHS, upon recognition of the handover, the candidate base station confirms that the mobile station is the correct mobile station by performing a proving test with respect to the mobile station which is a sender of the handover request. However, such a proving test requires a large amount of data processing in the candidate base station since a database, which is previously and systematically constructed with respect to each of the mobile stations accommodated in the system, must be referred to.
Additionally, in the PHS, there is a high-possibility that the communication service is interrupted at a high frequency when the mobile station, which is an object of the handover, is moving at an extremely high speed since the PHS inherently has a small diameter cell and a reuse of a radio frequency is positively attempted. However, such a reduction in the cell diameter and reuse of the radio frequency is effective especially for positive relief from a dead zone in the city area and for positively and economically forming a cell which can maintain a high quality transmission characteristic.
Accordingly, when the reduction in the cell diameter and the reuse of the radio frequency are attempted in mobile communication systems other than the PHS, there is a high-probability that the communication service is interrupted in the same manner.